Cane sugar, plus the water of hydrolysis, will yield theoretically 53.8 per cent of alcohol and 51.5 per cent of carbon dioxid.
In practice the theoretical proportions of alcohol and carbon dioxid are not obtained because of the difficulty of excluding other fermentative action, resulting in the formation especially of succinic acid and glycerol. Moreover, a part of the sugar is consumed by the yeast cells to secure their proper growth and development. In all only about ninety-five per cent of the sugar can be safely assumed as entering into the production of alcohol. About 48.5 per cent of alcohol are all that may be expected of the weight of dextrose or invert sugar used. Only sugars containing three molecules of carbon or some multiple thereof are fermentable. Thus the trioses, hexoses, nonoses, etc., are susceptible of fermentation, while the tetroses, pentoses, etc., are not.
159. Estimating Alcohol.—In the determination of sugar by fermentation, a rather dilute solution not exceeding ten per cent should be used. A quantity of pure yeast, equivalent to four or five per cent of the sugar used, is added, and the contents of the vessel, after being well shaken, exposed to a temperature of from 25° to 30° until the fermentation has ceased, which will be usually in from twenty-four to thirty-six hours. The alcohol is then determined in the residue by the methods given hereafter.
The weight of the alcohol obtained multiplied by 100 and divided by 48.5, will give the weight of the hexose reducing sugar which has been fermented. Ninety-five parts of sucrose will give 100 parts of invert sugar.
Example.—Let the weight of alcohol obtained be 0.625 gram. Then 0.625 × 100 ÷ 48.5 = 1.289 grams, the weight of the hexose, which has been fermented; 1.289 grams of dextrose or levulose correspond to 1.225 of sucrose.
160. Estimating Carbon Dioxid.—The sugar may also be determined by estimating the amount of carbon dioxid produced during the fermentation. For this purpose the mixture of sugar solution and yeast, prepared as above mentioned, is placed in a flask whose stopper carries two tubes, one of which introduces air free of carbon dioxid into the contents of the flask, and the other conducts the evolved carbon dioxid into the absorption bulbs. In passing to the absorption bulbs the carbon dioxid is freed of moisture by passing through another set of bulbs filled with strong sulfuric acid. During the fermentation, the carbon dioxid is forced through the bulbs by the pressure produced, or better, a slow current of air is aspirated through the whole apparatus. The aspiration is continued after the fermentation, has ceased, until all the carbon dioxid is expelled. Towards the end, the contents of the flask may be heated to near the boiling-point. The increase of the weight of the potash bulbs will give the weight of carbon dioxid obtained. A hexose reducing sugar will yield about 46.5 per cent of its weight of carbon dioxid. The calculation is made as suggested for the alcohol process.
The fermentation process has little practical value save in determining sucrose in presence of lactose, as will be described in another place.
161. Precipitation of Sugars in Combination with the Earthy Bases.—The sugars combine in varying proportions with the oxids and hydroxids of calcium, strontium and barium. Sucrose especially, furnishes definite crystalline aggregates with these bases in such a way as to form the groundwork of several technical processes in the separation of that substance from its normally and abnormally associated compounds. These processes have little use as analytical methods, but are of great value, as mentioned, from a technical point of view.
162. Barium Saccharate.—This compound is formed by mixing the aqueous solutions of barium hydroxid and sugar. The saccharate separates in bright crystalline plates or needles from the warm solution, as C₁₂H₂₂O₁₁BaO. One part of this precipitate is soluble in about forty-five parts of water, both at 15° and 100°.
163. Strontium Saccharates.—Both the mono- and distrontium saccharates are known, viz., C₁₂H₂₂O₁₁SrO + 5H₂O and C₁₂H₂₂O₁₁2SrO.